Puzzled by a star that seemed to stay too bright in the sky too long after it had exploded, astronomers turned to their ultimate gumshoe, the Hubble telescope. Now they think they have solved the mystery. At a meeting of the American Astronomical Society in Winston-Salem, North Carolina, researchers announced today that they believe they have for the first time witnessed the aftermath of a collision between two supernovas.

Examining a bright object 17 million light-years away in the constellation Cepheus a couple years ago, astronomers William Blair of Johns Hopkins University, Robert Fesen of Dartmouth College, and Eric Schlegel of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, at first thought they were peering at a young supernova. "It was so bright in the x-ray and so bright in the optical, we thought it was very young, maybe 100 years old," says Blair. But they soon realized that even a young supernova shouldn't be so bright. "The fact that it was so many times more luminous than what we expected set up flags that it couldn't possibly be a normal supernova," says University of California, Los Angeles, astronomer Schuyler Vandyk.

Indeed, when the researchers measured the speed of the gas cloud thrown off from the supernova explosion, they found it was moving at a rate that implied the star had exploded thousands of years earlier. Perplexed, the group trained the Hubble telescope on the light source. To their surprise, they discerned two objects--a pair of exploding stars, roughly 40 light-years apart. The shock waves of expanding gas from each star had collided, compressing the shells and providing a stellar fireworks display.

"It's very interesting," says Vandyk, who says that a double supernova lends support to a theory that supernovas carve out supershells, large regions in galaxies sparsely populated with stars. But such an explanation hasn't satisfied all the experts. "I kind of suspect it's not the full story yet," says astronomer Frank Winkler of Middlebury College in Vermont. "I'd certainly like to know more."